Generally, in plasma processing systems photons, ions, and other plasma particles impinge on a wafer and heat the wafer up. For plasma processing, the wafer is placed on an electrostatic chuck in a processing chamber. Typically, a gas (e.g., helium) is used on the back side of the wafer to enhance heat transfer between E chuck and the wafer. To introduce the gas with less resistance, grooves are milled into the chuck. The gas entering the grooves on the chuck diffuses under the wafer and can leak beneath the wafer into the chamber.
Typically, only a minor portion of helium back side gas (e.g., at a flow rate of about 0.5 standard cubic centimeters per minute (SCCM) passes through the chuck due to good seal between the polished outer seal band on the chuck and the back side surface of the wafer. The major portion of the backside helium (at a flow rate of about 19.5 SCCM) is dumped through an orifice in a vacuum system. This is not an efficient way of using the back side expensive heat transfer gas.
Currently, due to government regulations and increased cost of helium, many manufacturers are using gases other than helium, such as nitrogen and argon as back side gases in electrostatic chucks. Nitrogen and argon have serious limitations in electrical ionization potential and thermal properties that can be unacceptable for some plasma tools.
Furthermore, argon and nitrogen back side gases at certain plasma conditions have conductive path and arcing issues. These issues lead to generation of defects (e.g., holes, marks, other defects) and damaging the wafer that significantly limits the plasma processing design and increases manufacturing cost.